PHYSIOLOGICAL RETINAL PROCESSES. 



1047 



not hinder regeneration. When sufficiently intense, it acts, however, on 

 visual purple, and will in time completely bleach the retina. That 

 yellow light is also of very slight influence, is shown by the use of the 

 sodium flame in experimental investigations on visual purple ; its action, 

 however, is greater than that of red light. Visual purple is most rapidly 

 bleached by the part of the spectrum between D and E, and from the 

 maximum of action in this part of the spectrum there is a gradual 

 diminution with diminishing wave-length. Kiihne investigated whether 

 the slighter activity of the violet end was due to greater spreading out 



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Fig. 380. 



of that end in the prismatic spectrum and consequent smaller intensity, 

 but experiments with a grating spectrum also showed the greatest activity 

 of the yellow-green rays. The degree of activity corresponds with the 

 degree of absorption of monochromatic light by visual purple (Fig. 379). 

 Visual yellow is acted on more strongly by the violet end in correspond- 

 ence with its absorption spectrum, and consequently visual yellow is 

 often absent as an intermediate product when visual purple is bleached 

 by light of short wave-length. Fuchs and Kreidl x have found that the 

 Eontgen rays have no influence on the visual purple of the frog, and 

 Pergens 2 found that this was also the case after removal of the lens in 

 Leuciscus rutilus. Gatti 3 has found that the rays have no influence on 

 the regeneration of visual purple. Fuchs and Kreidl found, on the other 

 hand, that the yellow-green fluorescent light from the tube bleached 

 visual purple very slowly. 



The whitish-green fluorescence of the retina in ultra-violet light depends on 

 the rods and on the bleached visual purple. The unbleached rod layer 

 fluoresces very slightly and with a bluish tint ; when completely colourless, 

 the fluorescence is much stronger and greenish in appearance. Visual yellow, 

 on the other hand, appears to be completely devoid of fluorescence. The cones 

 and the rods about the ora serrata, which have no purple, do not fluoresce, and 

 under ultra-violet rays the fovea appears as a dark spot. It is difficult to 

 determine whether the solution of visual purple is fluorescent, since the bile in 

 which it is dissolved fluoresces strongly (Ewald and Kuhne). 4 



The function of visual purple will be considered later in connection 

 with the theories of light and colour vision. It will be sufficient to 



1 Centralbl.f. Physiol., Leipzig u. Wien, 1896, Bd. x. S. 249. 

 "Klin. Monatsbl. f. Augenh., Stuttgart, 1897, Bd. xxxv. S. 354. 



3 Centralbl.f. Physiol., Leipzig u. Wien, 1897, Bd. xi. S. 461. 



4 Untersuch. a. d. physiol. Ins/, d. Univ. Heidelberg, Bd. i. S. 139. 



